Overview

While normally it's said that every atom wants a complete octet of electrons to fill its valence electron shell, this is not entirely true. Almost every rule in science has exceptions, and this rule has many. In the valence shell of electrons, something called hybridization, where different orbitals of the atom combine to form hybrid orbitals, that each have one electron. There by, an atom can have up to 6 bonds, giving it a total of twelve electrons, and violating the so-called octet rule. However, the atoms is not really cheating. What it has done, is boosted the electrons in each orbital to higher orbitals, so that the electron is free to bond with another atom. Take carbon for example. carbon has 4 electrons in its valence chell, two in the s sublevel, and 2 in the p sublevel. As per Hund's Rule, electrons fill each orbital in the sublevel once before they begin pairing up. Therefor the p sublevel has two orbitals with one electron each. This SHOULD mean that carbon can only form two bonds. After all, the two electrons in the s sublevel are happy: they have each other. However, we know this is not the case, carbon can form up to four bonds.

This is where hybridization comes in. One of the electrons in the s sublevel gets boosted to the empty orbital in the p sublevel, giving it four orbitals with one electron each, and they want a mate. What you end up with is four orbitals that is a combination of the s sublevel/orbital, and each of the p orbitals involved, called sp3, because there were 3 p orbitals involved and an s orbital.

Orbital Types and Examples

sp: When just one s orbital and a p orbital is involved, an sp orbital is created. An example is berylium, who has two electrons in its s sublevel. It should be happy and not make bonds. However, it boosts one of its electrons to the p sublevel, and ends up with 2 sp orbitals, ready and willing to bond.

sp2: One s orbital and two p orbitals. An example is boron. Once again, it should one make one bond because of a happy s orbital. However the one s electron is boosted, giving it three sp2 orbitals.

sp3: An example has already been given, using carbon.

Now we've reached the point where all the p orbitals are filled. What do we do now? Well, if you're nitrogen and oxygen, nothing. That's all the hybrids you can make. However, if you're in the third level and above, namely sulfur and phosphorus, you can make additional hybrids, by promoting electrons to the d sublevel. Normally the d sublevel remains unfilled for these atoms, however once again science has many exceptions, and these are but a few.

sp3d: One s orbital, three p orbitals, and a d orbital are involved in making this. An example is phosphorus, which can make up to 5 hybrid orbitals using this technique.

sp3d2: One s orbital, three p orbitals, and two d orbitals, this is the largest amount of hybrid orbitals an atom can make, 6 of them. Sulfur is a prime example, take a look at SF6, impossible under the octet rule, but in real life existing. How? Each electron seperates and pairs up forming a bond.

Shapes

Scientists have a specific shape for each electronic configuration of the atoms, or where the bonds are located. In some cases, there is simply a long pair of electrons in the place of where the bond should be. In that case the molecular geometry is different from the electronic geometry, and scientists have different names for that too.

sp2: The bonds point to the tip of a triangle, giving it the name Trigonal Planar, because the triangle is on one plane. 120 degree angles between the atoms.

sp3: The bonds point to the tips of a Tetrahedron, a three-dimensional shape that has an angle of 109.5 degrees between each atom.

sp3d: The bonds point to the tips of a shape called a trigonal bipyramidal, which can be envisioned thus: Picture a pyramid with three sides, now place that pyramid's base flat against the base of another pyramid. There is a 90 degree angle between the tips of the pyramids and the atoms at the base, and a 120 degree angle between each of the atoms at the base.

sp3d2: The bonds point to the tips of an octahedron. To envision: Think of a pyramid with a square base, then put another tip below the base. It's called an octahedron because it has a total of 8 sides. The octahedron has all 90 degree angles, the central atom being the vertice, and outside atoms being the tips of the angle.

JerboaKolinowski has taken the time to point out that all angles are based on the point of view of the central atom, and not from arbitrary atoms. All should now /msg him thank yous, as many times as you can.